Hello All.

I didnot find a suitable answer for this thing in search. This topic was discussed in 2001 OR 2000, but did not discuss to this extent. The discussion ended before these details. If anyone can find the answer, please forgive me and tell me the link.



I am using AT89C51 to read the RPM of the car.

Here is what I do:

Use the interrupt pin (and ISR) to time the duration of one neg-edge to another. Use a formaula to convert the pulse width to Engine RPM. Then I show the result on a display.

I am using a transistor to translate the 12V signal from the ignition coil to a 5V signal for the uC.

My problem:

As long as I don't touch (any of the) primary connections of the ignition coil and HOLD the wire close to the ignition coil instead, I get the correct reading (verified by an auto mechanic's gadget).

The moment I touch the ignition coil primary, my display goes crazy.

I have tried a capacitor in series to limit the pulse width, but it did not work.

I am definitely missing some filtering. Here is my ignition coil interface.

A wire from primary of ignition coil through a 22K resistor goes to base of a transistor. Emitter is grounded. Collector is connected to +5V through a 1K resistor. +5V is obtained using LM7805. Everything gets supply from car battery. The collector is connected to INTx pin of the uC.

Back in the previous discussion, people were talking about using a separate battery. Do you think using a separate battery would solve the problem ??

Regards
Aman